1. Field of the Invention
The present invention relates to a method for regulating, for the purpose of burning in a recovery boiler furnace for soda-ash, the feeding-in and/or combustion conditions of concentrated spent liquors of varying chemical and physical composition by measuring some physical property of the liquor to be fed into the soda-ash furnace and by regulating the feeding-in and/or combustion conditions directly on the basis of the property thus measured.
This invention relates in particular to a method for regulating, for the purpose of burning in the recovery boiler furnace, the feeding-in and/or combustion conditions of liquor mixtures which contain, in varying proportions, liquors derived from both sulfate and sulfite digestion processes.
2. Description of the Related Art
As is known, spent cooking liquor is produced in pulp digestion, and for the economy of pulp production it is very important that the heat content and chemicals of this spent liquor are recovered as carefully as possible for reuse in the pulping process. Before the spent liquor is burned in order to release the thermal energy and to recover the chemicals, water is evaporated out from the liquor to produce a liquor which contains about 40% water; this concentrated liquor is burned in the recovery boiler furnace and the thermal energy thereby released can be used in the pulping process, and the chemicals can be recovered from the bottom of the furnace and, after regeneration, can be used for the preparation of new cooking liquor.
As energy prices have risen continually, it has become increasingly important for the economy of the pulping process that the burning of liquor in the recovery boiler furnace is as disturbance-free as possible in order to achieve good chemical economy, low emissions, high energy efficiency, and good process economy.
For the primary function of the recovery boiler furnace, i.e. the recovery and regeneration of inorganic chemicals for the preparation of cooking liquor, it is necessary to create in the lower part of the soda-ash furnace a reducing section having a high temperature and a so-called char bed at its bottom. The degree of regeneration in the furnace is measured in terms of sulfur reduction degree.
The recovery of the chemicals is measured in terms of loss of chemicals. Losses are incurred when gases such as SO.sub.2 are emitted from the process along with flue gases.
Another function of the recovery boiler furnace is to recover heat from the flue gases. The effectiveness of this recovery can be measured in terms of the flue gas losses, the proportion of unburnt gases, and the usability of the furnace, for example, in terms of stoppages due to the fouling of the heating surfaces.
The operation of the recovery boiler furnace is affected by many factors. The concentrated spent liquor fed into the furnace still contains a relatively high amount of water (about 40%). This water amount must be caused to evaporate in the furnace, and the evaporation must take place substantially from the liquor droplet falling towards the char bed at the bottom of the soda-ash furnace, before the droplet reaches the surface of the char bed. If this does not happen, a large proportion of the water must be evaporated from the surface of the char bed, which of course decreases the temperature of the char bed and, in turn, increases the emission of sulfur dioxide and decreases the reduction degree.
If water has evaporated before the droplets reach the char bed, the droplets become so light in weight that they may be captured by the gas flow rising in the furnace, whereupon they are pyrolyzed and burned in suspension (flow), consequently increasing the dust load in the gas flow. The aim is to make the size of the liquor droplets in the furnace such that the dry matter content is suitable at the time the droplet hits the surface of the char bed and that the remaining small amount of water evaporates rapidly from the surface of the char bed and produces a porous char bed. Thereby the char bed at the bottom of the furnace remains hot, making it possible to maintain good chemical economy and good usability of the furnace.
Droplet size suitable in terms of the operation of the recovery boiler furnace has been determined visually on the basis of experience, for example by observing the temperature of the char bed, at the bottom of the recovery boiler furnace on the basis of, for example, color or by measuring. It has been noted that it is the viscosity of the liquor fed into the furnace that primarily determines the size of the droplets formed in the gas space of the recovery boiler furnace when, for example, the size and type of the nozzles feeding liquor into the furnace, as well as the feeding pressure, remain substantially constant. Respectively, when the viscosity is constant, the droplet size is determined by the nozzle diameter at a constant flow of liquor.
In order to maintain the droplets at the size experimentally found to be good in the above-mentioned manner, the quantity used as the control parameter has been the dry-matter content of the concentrated liquor, determined by means of its density or by using a refractometer, and the changes to be affected in the temperature and the injection pressure of the liquor fed into the furnace in order to obtain droplets of the desired size in the gas space of the furnace have been determined on the basis of the measurement. In the main, the viscosity of the liquor has been regulated by heating the liquor. Such regulation is described in the publication Pulp and Paper 53, (1979), pp. 142-145.
Aerometric measuring is commonly used for measuring the density. The raw material and the cooking conditions remaining constant, dry-matter measuring by means of a refractometer yields a quantity which can be used for the control of the recovery boiler furnace.
Disturbance-free operation of the recovery boiler furnace was achieved previously by maintaining the preparation process and, consequently, the properties of the concentrated liquor as even as possible, and owing to this it was possible to operate the burning process at a constant setting. Previously, pulp mills used in general one single type of wood in each mill, and, likewise, usually one single specific pulp type was produced, and, as a result, the chemical composition of the spent liquor remained more or less unchanged.
The operation of the evaporation plant was adjusted in such a way that a certain maximally constant dry-matter content was reached, and the burning process was adjusted according to this content. Efforts were made to regulate the dry-matter content with a precision of about .+-.1.5 percentage points. If fluctuations are great, they reflect in the operation of the recovery boiler furnace, causing changes in the degree of reduction, SO.sub.2 gas emission, and fouling of the furnace. Whenever difficulties have appeared, the operator of the recovery boiler furnace has requested that a check be made whether the process parameters in the evaporation plant and in the digestor have remained within the set range.
Fluctuations in the chemical composition of the liquor to be burned are caused by increasingly closed processes, i.e. closed chemical cycles. Variations in the raw materials also require new digester parameters, a factor which complicates the operation of the evaporation plant. Furthermore, the liquors of an increasing number of cooking processes are burned in one and the same furnace. Under these circumstances the properties of the liquor cannot be maintained as constant as previously.
When mixtures of parallel cooking liquors are burned and when other waste materials are added to the liquor to be burned, the disturbance is shifted directly to the recovery boiler furnace.
In addition the above-mentioned major disturbances at the recovery boiler furnace, the overall quality standards set for the equipment have risen. There is a high requirement for usability under varying conditions, while the SO.sub.2 level in the flue gases and the degree of reduction in the smelt must be at controlled levels.
The feeding into the recovery boiler furnace of concentrated spent liquors of varying chemical and physical composition so as to produce a suitable droplet size in the furnace has been regulated by adjusting the feeding-in conditions of the liquor being fed into the recovery boiler furnace, not only on the basis of the above-mentioned dry solids content measured from the concentrated spent liquor but alternatively also on the basis of a viscosity value measured directly from the liquor fed into the recovery boiler furnace. Using viscosity measurements as the control quantity for the feeding in of liquor is a much more rapid and more simple method then regulating the recovery boiler furnace on the basis of a dry solids analysis. On the basis of viscosity measurements it is possible to adjust the feeding-in conditions rapidly to such values that the liquor discharged through the nozzles forms droplets of the desired size.
It has, however, now been observed that, although on the basis of viscosity measurements it is possible to adjust the feeding-in conditions to such values that the liquor discharging from the nozzles can be caused to form droplets of the desired size, it is not possible on the basis of this viscosity measurement to determine how the liquor droplets thus formed behaves while falling in the furnace space towards the surface of the char bed at the bottom of the furnace. It has been observed that a change in the chemical or physical composition of the liquor may cause a change in its combustion behavior in the recovery boiler furnace even if the viscosity of the liquor, and thereby the size of the droplet formed by the liquor discharging from the nozzle, remains the same. From this it has been concluded that some unforeseen factor influences the combustion behavior of the liquor drop falling in the gas space in the recovery boiler furnace, in which case the above-mentioned methods of measuring are not sufficient for regulating the size of the liquor droplet formed in the gas space of the recovery boiler furnace when the chemical and physical properties of the dry matter in the liquor being fed into the furnace vary, for example because the type of wood or the method used in the pulping process, or the batching and additions of chemicals have been changed.